A power device is widely used as a switching device for various types of power supplies or various types of electrical devices of an automobile, or used as a switching device of an electrical device of industrial machinery. As compared to a typical semiconductor device, the power device is configured to have characteristics of a high voltage, a high current, a high speed and a high frequency. Examples of such a power device may include an IGBT, a diode, a power transistor, a power MOS-FET, a thyristor, and so forth. These power devices are put on a market as electronic components for individual purposes after static characteristics or dynamic characteristics (e.g., a switching characteristic) thereof are evaluated.
A diode is used as a switching device of, e.g., a motor by being connected in parallel to a power MOS-FET, for example. As for a switching characteristic of the diode, it is desirable that a reverse recovery time of the switching device is short. If the reverse recovery time is long, the diode may be damaged depending on conditions for the usage thereof. Further, a sharp current variation (di/dt) of a reverse current may cause an increase of a current, so that the diode may be easily damaged. To measure the switching characteristic (dynamic characteristic) of the power device, every single package product of power device is measured by a dedicated measuring device, and reliability as an individual power device is evaluated.
If, however, the package product is found to be a defective product, the package product is disposed of, resulting in high cost for a qualified product. To reduce such a waste, the present applicant conducted various ways in which the power device is inspected on a wafer level by using a probe apparatus. The probe apparatus used when inspecting the power device includes a movable mounting table configured to mount thereon a semiconductor wafer; a probe card provided above the mounting table; an alignment device configured to align the semiconductor wafer with the probe card in cooperation with the mounting table; and a tester provided on and electrically connected with the probe card. The probe apparatus is configured to evaluate a dynamic characteristic such as a switching characteristic of a power device by bringing probes of the probe card into electric contact with electrodes on the semiconductor wafer after the semiconductor wafer and the probe card are aligned with each other and, then, by measuring, e.g., a current variation of the power device.
For example, a gate electrode and an emitter electrode are formed on a top surface of a semiconductor wafer on which a multiple number of power devices are formed, and a collector electrode is formed on a bottom surface of the semiconductor wafer.
In the probe apparatus configured to evaluate the dynamic characteristic of the power device, a collector electrode film made of a conductive film is formed on a top surface of the mounting table to be brought into contact with the collector electrode of the power device. Typically, the collector electrode film and the tester are connected by a cable.
In the conventional probe apparatus, however, since the cable that connects the collector electrode film of the mounting table and the tester is long, an inductance of the cable is increased. For example, an inductance of about 100 nH may increase per every 10 cm of the cable. If a current variation (di/dt) is measured on a microsecond level by using such a probe apparatus, the current variation, from which the dynamic characteristic of the power device would be evaluated, may be decreased and deviated far from an ideal value. Accordingly, it becomes difficult to measure an actual current variation (di/dt) accurately, and, occasionally, the power device may be damaged. For these reasons, the conventional probe apparatus is found not to be capable of accurately evaluating the dynamic characteristic such as the switching characteristic of the power device. Further, when turning off the power device, an abnormal surge voltage may be applied between the collector electrode and the emitter electrode, so that the power device may be damaged.
In view of the aforementioned problems, the present applicant investigated various ways to suppress an increase of the inductance in the cable. As one solution, the applicant proposed a probe apparatus shown in FIG. 7A and FIG. 7B (see Patent Document 1). In this probe apparatus, a special conducting unit is provided instead of the cable that connects the mounting table and the tester. This probe apparatus will be briefly explained with reference to FIG. 7A and FIG. 7B. As illustrated in FIG. 7A, the probe apparatus includes a mounting table 2, a probe card 3 and a conducting unit 4 in a prober chamber 1. A conductive film made of a conductive metal such as gold is formed at least on a top surface of the mounting table 2 as a collector electrode. Above the mounting table 2, the probe card 3 having a multiple number of probes 3A is fixed at a head plate (not shown) of the prober chamber 1 via a card holder 5 (see FIG. 7B). Terminal electrodes corresponding to the multiple number of probes 3A are formed on a top surface of the probe card 3 in a certain pattern, and the multiple number of probes 3A are electrically connected with a tester (not shown) via the respective terminal electrodes. For example, in FIG. 7A and FIG. 7B, a left probe 3A may come into contact with a gate electrode of a power device, and a right probe 3A may come into contact with an emitter electrode of the power device. By applying a voltage to the gate electrode, a current flows from a collector electrode to the emitter electrode of the power device, and a current variation (di/dt) at this moment is measured.
Furthermore, as depicted in FIG. 7A and FIG. 7B, the conducting unit 4 configured to electrically connect a conductive film electrode of the mounting table 2 and the tester is provided at the mounting table 2, the probe card 3 and the card holder 5. The conducting unit 4 includes, as shown in FIG. 7A and FIG. 7B, a pair of connecting terminals 4B provided at positions on a peripheral surface of the mounting table 2 such that the connecting terminals 4B face each other; and a pair of segment conductors (contact plates) 4C provided between the mounting table 2 and the probe card 3 to correspond to the pair of connecting terminals 4B. Even if the mounting table 2 is moved toward any position to measure an electrical characteristic of each power device, each of the pair of connecting terminals 4B may come into elastic contact with a corresponding one of the contact plates 4C. As a result, the collector electrode film and the tester (not shown) can be electrically connected to each other.
In the probe apparatus shown in FIG. 7A and FIG. 7B, since the conducting unit 4 is provided as stated above, a line length between the collector electrode film of the mounting table 2 and the tester is remarkably shortened, and, accordingly, an inductance is reduced. Thus, when evaluating the dynamic characteristic such as the switching characteristic of the power device, it is possible to measure a current variation in the power device accurately.